Hibernate: childEntity null id on merge parentEntity

Question

I've the code:

modifiedParentEntity.addChild(newChildEntity);
session.merge(modifiedParentEntity);
newChildEntity.getId(); // == 0!!! 

Where parent entity has a collection of child entities with CascadeType.ALL:

@Entity
public class ParentEntity {
    @OneToMany(cascade = CascadeType.ALL, fetch = FetchType.EAGER, orphanRemoval = true)
    @JoinColumn(name = "parent_entity_id")
    private List<ChildEntity> childEntities;
}

Question:

1. Why after session.merge(modifiedParentEntity) chidEntity has 0 id?
2. How to save both modifiedParentEntity and chidEntity in a way that Hibernate will set id to chidEntity?

P.S.:

I use postgres and sequences to generate entities' ids.

Answer 1

merge operation does not make the passed entity managed, but rather returns another instance that is managed

so you need to get the id from managed instance as follows

modifiedParentEntity.addChild(newChildEntity);
ModifiedParentEntity modifiedParentManaged= session.merge(modifiedParentEntity);
session.flush()

Then get the child entity from modifiedParentManaged and then get its id.